Method and means for operating an airborne switched array radar system

ABSTRACT

An airborne radar system is disclosed utilizing multiple fixed antenna arrays mounted within the periphery of the aircraft to avoid aerodynamic modifications and optimumly placed to achieve 360* surveillance coverage. The arrays preferably include a fore mounted array, an aft mounted array, a port mounted array and a starboard mounted array for respectively firing beams in different azimuth sectors relative to the aircraft. Each array is comprised of dipole elements having phase shifters coupled thereto for steering a beam within the corresponding sector. The primary radar antennas time share an exciter, transmitter, receiver and signal processor through switching devices. Time allocation between antennas and between operational modes such as &#39;&#39;&#39;&#39;search&#39;&#39;&#39;&#39; or &#39;&#39;&#39;&#39;track&#39;&#39;&#39;&#39; is based on various factors such as mission objectives, current target characteristics and radar purpose. Time allocation is preferably determined by an &#39;&#39;&#39;&#39;on line&#39;&#39;&#39;&#39; stored program digital computer which generates a radar control command to define the parameters for each beam to be fired. This procedure involves calculating the priority value of each track and search beam to be fired in accordance with predetermined criteria contained in the stored program. The priority value of each track beam to be fired involves determining the update rate for that target which is calculated based on the characteristics of the target. Priority values are then determined based on a comparison between the elapsed time since the last update and the calculated update rate.

United States Patent [191 Scheidler et al.

[451 Dec. 31, 1974 METHOD AND MEANS FOR OPERATING AN AIRBORNE SWITCHED ARRAY RADAR SYSTEM [75] Inventors: Stuart P. Scheidler, Anaheim;

Gerald M. Goldberg, Chatsworth; Richard Sidlo, Placentia; Donald L. King, La Mirada; Richard A. Gebhardt, Orange, all of Calif. [73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Feb. 5, 1973 [21] Appl. No.: 329,765

[52] US. Cl. 343/7 A, 343/5 DP [51] Int. Cl. G015 9/02 [58] Field of Search. 343/5 DP, 7 A

[56] References Cited UNITED STATES PATENTS 3,460,137 8/1969 Ralston 343/7 A X 3,699,573 10/1972 Andrews et al.... 343/7 A X 3,757,326 9/1973 White 343/7 A Primary Examiner-T. H. Tubbesing Aztorney, Agent, or F irmW.-H. MacAllister; Walter J. Adam [57] ABSTRACT An airborne radar system is disclosed utilizing multiple fixed antenna arrays mounted within the periphery of the aircraft to avoid aerodynamic modifications and optimumly placed to achieve 360 surveillance coverage. The arrays preferably include a fore mounted array, an aft mounted array, a port mounted array and a starboard mounted array for respectively firing beams in different azimuth sectors relative to the aircraft. Each array is comprised of dipole elements having phase shifters coupled thereto for steering a beam within the corresponding sector. The primary radar antennas time share an exciter, transmitter, receiver and signal processor through switching devices. Time allocation between antennas and between operational modes such as search or track is based on various factors such as mission objectives, current target characteristics and radar purpose. Time allocation is preferably determined by an on line" stored program digital computer which generates a radar control command to define the parameters for each beam to be fired. This procedure involves calculating the priority value of each track and search beam to be fired in accordance with predetermined criteria contained in the stored program. The priority value of each track beam to be fired involves determining the update rate for that target which is calculated based on the characteristics of the target. Priority values are then determined based on a comparison between the elapsed time since the last update and the calculated update rate.

12 Claims, 24 Drawing Figures Prim-1y Ildll nmll DIVIMOI nl ui computer lvillm Cnmmumtallon! 50 am Syllun Y WARNING SYSTEM PAIENTEDHEEQ I m4 3.858.206 sum 01 [1F 18 Aircraft horizontal PAIEHTEI] UEC3 1 i974 SHEEi (320i 18 Communications system Mission avionics system Fig. 2a,.

30 Radar System AIRBORNE EARLY WARNING SYSTEM I Primary 46 radar a subsystem Port 3 4 48 Stbd Fore 5O Aft Recvr Xmitter Exciter 6O 44 Primary radar Beam signal processor wring l array 40 switch Radar control unit Digital computer system Secondary radar Beam signal processor 72 steering 77 array 32 7 4 switch 76 i f Recvr lnterrogator Port , I Stbd Fore 68 Aft 36 70 Secondary I radar subsystem l A Pmmmnm 1 4 I 3'. 858.206

sum 03 0F 18 Fig. 2c.

87 8 Primary radar SLB azimuth feed w Module1 86 2 i N 84 88 I j I y I Indicates a 89 horizontally T I h Wlarized Secondary radar SLB azimuth feed w element Indicates vertically 9 polarized element F g. 2b.

Module1 2 N Secondary radar azimuth feed Primary radar azimuth feed To high power 4:1 switch Q Phase shifter PATENTED EH13] I974 SHEET 0 40f 18 PRIMARY RADAR SUBSYSTEM Fig. 3G.

Port Stbd Fore Aft Radiating SLB A Dipoles 84 Antenna Low power Dmde 93 1:4 switch Phase Shifters Feed Structures High Power 1:4 sLe Switch/Duplexer Hammer 6O 40 F f r 56 I I I r 1 Radar Transmitter Receiver control Digital l Processor unit computer I v Exciter PATENTED DECS 1 IBM i 3858.206 sum USUF i8 Fore antenna X X Receiver output Port antenna Starboard antenna X X X Aft 98 antenna Fig. 5b.

Transmitter input MTI double canceller F 1st Diff. 4.x 2nd Diff.

I (B n c I C I56 I v n From A/D I converter (A 8 (B C 10 bits s|gn Subtracter Subtracter bits sign I50 8 I54 Sweep 1st Diff. storage A store n n n F To 8 bits doppler sign accumulator Llmiter PATENTEB flEC3 1 I974 SHEET 069E l8 32 Digital computer 40 Radar I control [3-6 unit Back up scan program Manual switch /l3O lnp ut Radar data logic buffer A Receiver 4 Exciter Read Oul Radar Enable Antenna synchronizer Computer to Fig.6. High power 4 switch 91 I Exciter Array switches Phase shifter Signal processor I02 06\ H2 (I I6 /||8 I20 I l l l l l Double Blank MTI Adaptive Coherent Noncoherent h shold I04 I08 integrator integrator and filter i selection Q Double- A/D Blank MT M4 P w A/D Doppler Processor llO f SLB L recvr I00 2 8 H 1 Clear Recvr 8 M l i channel sensor 5e Fig. 4

PATENTEUDEE31 m4 SHEET 070F 18 PATENTEU DECS 1 I974 From computer SHEET O8UF 18 For next dwell M (mode) radar control command M. F. ID. EL, Az,

PRF

To computer beam return report F (frequency) ID (array ID) EL (sin elevation angle) A: (sin azimuth angle) Exciter Antenna j phase computer PRF (pulse repetition freq) B (burst no.) FP (fill pulses) Radar synchronizer TH (threshold) R start (range cell start) R stop (range cell stop) A F (ground doppler offset) Recerver/ signal processor For this dwell CL (clutter level) CT (subreport count) J (jam level) 'Fl (range) F no. (filter no.) S/C+N (signal/clutter Noise) A (amplitude) M Signal processor Fig. 7a.

RADAR DATA BUFFER PATENTEBDEB31 I974 maw-c PRF R start R stop SHEEI UQUF 18 start stop YPRF

(array ID) 00 Fore 01 Strbd (mode) 0 Search 1 Verify 2 Track (sine of A2 angle) sin .088

(sine of EL angle) sin .088

(threshold) .5 db

(frequency) (gnd doppler offset) (pulse repetition freq) (fill pulses) (no. of bursts) (range oell start) (range cell stop) IPPS 1 pulse 1 burst Fig. 7

10=Aft 11 Port 4 Test 5 Sea State 3, 6 and 7 not defined i sin 55 i sin 93 i 4000 Hz RADAR CONTROL COMMAND FORMAT Bits PATENTEU DEBS 1 I974 mumm-hwivo SRCT PRF

SHEEI lOUF 1s EL ncr Header Report Header (one per dwell) (frequency) 0 31 5 (sine of Al angle) sin .088 i sin 55 12 (sine of EL angle) sin .088 i sin 9.8 12 (elutter level) 0 31 5 (array ID) 00 Fore 10 Aft 2 01 Strbd 11 Port (sub-report count) I I 0 63 6 (mode) 0 Search 4 Test 3 1 Verify 5 Sea State 2 Track 3, 6 and 7 not defined (jam level/frequency) O 7 3 (pulse repetition frequency) IPPS 1562-3125 12 Report (N" per dwell) (ambiguous range) lRC 0 5000 13 (signal/clutter noise) 2 db 8 40 db 4 (amplitude) 3/8 db O db 9 (filter number) 0 15 4 F i g 7 c.

BEAM RETURN REPORT FORMAT PATENTED l l 3.858.206

SHEET 1 1 0F 18 2 3 Clock CL z Res Radar data CL Counter buffer 26 6 274 l L TSUD 6 T Fladar syne 27 LPS V250 275 ees 1 Register 278 Delay load I I SDO Sweep T internal SDI 252 reglster 280 T 297 295 Parallel Delay 294 enter T CL s I DEC Counter 292 Ts 298 290 l Delay 254 3l2/ T 306 302 258 DEC FP 0 T register QL Delay SP SDi D E F 300 BIO DEC FP+16B #0 308 T register 0 T o 260 320 256 scale of ECP 16 counter v 324 v l/ 323 3 Delay 262 SDi TS R start DA register 322 336 CL DEC O l TSDR Counter A l 332/ & 330

264 HE R stop register 340 o EDR CL DEC Counter y PNEhITEU J From synchronizer Data buffer Fig. 9.

Fig. 10.

sum mar 18 LPs sun To 5-bit phase shifter Element no. 1

----- Array switch s a Register Register Register PRF 3125 PRF 2232 PRF 1562 FP=13 FP=10 FP=8 (390 I 3 Stage Sin 0 Ring Counter counter LPs Radar data buffer Input beam return report parameters Fig. 116.

Call coordinate conversion routine Verify f 4 IO Add one to buffer pointer (mod 4) Record Call time of range measurement 4 I 2 resolution routine Call multipath ls resolution this an routine existing track 432 r422 Any Set verify tentative request can targets save target I t r 420 7 parameters n l gi'lze routine Compute Add one trick t r go ifi Add one up a e ra e (mod 4) v 442 to puffer pointer 424 (mod 4) AT=1.1 E 426 PArEmEunEm m 3,858,206

sum 15 0F 18 REM 400 Fig. 11b.

Energy Management 458 Is verify Yes REM 456 request VerIfy set 460 Compute T Tc TLU track priorities AT Select largest track priority 462 I5 it greater Search Compute PS T T se arc :h S pggmy T5 K N T REM 30 Track PATENTEU BEECH m4 Array routine REM 800 Call convert to radar coord routine Determine antenna array from azimuth Determine sine of target Az angle from broadside Determine sine of target EL angle from broadside Compute AF in frequency from R ground doppler offset Fig. 11c-1.

SHEET lBUF 18 Set mode to track array routine Add +Az and -Az for seq lobes Get range start and range stop from track store Call PR F and frequency routine Compute total pulses burst no. and fill pulses Compute detection threshold Store beams 1 and 2 at buffer pointer 1 and buffer pointer 2 mod 4 l to 5I2 PATENTED [1EE3 I I974 3.858.206 'SHEEI 17UF 18 Set mode set mode to search to verify 5 542 Call search pattern routine Call array routine 5 48 546 Retrieve 82;, search PR F routine and i jgg gg Assign the 3:2 3 standard Call PFlF st a 5000 PR F's and and frequency op frequencies selection 4 v routine 5 5 2 5 6O Assign standard Set range total pulses start 0 and compute and fill stop 5000 \l 524 total pulses pulses and fill 562 pulses Ass'gn Store beam total 1 at buffer y and Set range pointer 1 fill pulses 526 an and stop I r to max 564 maneuver 554 Compute gate detection threshold 52 Compute detection Store beams 565 threshold 1, 2, 3 at buffer pointer +1, +2, +3

mod 4 Store beam 1 at buffer pointer 1. 53 O I mod 4 from 5lO at buffer pointer 7 PATENTEU 3,858,206 SHEET 180F18 Eocessor (optionalL Memory 16K) j TI I Address/data to memory I w I Processor 1 6 I O L Memory (16K) Real time clock Priority interrupt register Interrupt network 628\ Address generator Y 627 w 622 Address buss Jr 629 I Control 6'3 unit Progran; counter I Data buss 4 selector To memory Shift I counter s20 Command i reglster B register Timing counter Contro Unit l 62| M Sense indicators 6 I 8 and switches Q register Data from memory Input/output Contro' lnterprooessor I l 1 2 module control Data from memory Channels 0 -3 Address/data to memory fCOI'IIIBCtIOIIS fir or Data from memory additional h l 2 s I Address/data to memory modules Control Data from memory Address/data to memory l J 600 l METHOD AND MEANS FOR OPERATING AN AIRBORNE SWITCIIED ARRAY RADAR SYSTEM CROSS REFERENCES TO RELATED APPLICATIONS This application is related to applications Ser. No. 329,762 and Ser. No. 329,763 both filed Feb. 5, 1973.

BACKGROUND OF THE INVENTION This invention relates generally to radar systems suitable for use in airborne early warning applications.

DESCRIPTION OF PRIOR ART Airborne radar systems useful for early warning applications are well known in the art. Generally, such systems utilize mechanically rotating antennas installed in dishes attached to the outside of the airborne platform. Such arrangements adversely effect the aerodynamic characteristics of the platform. Even more significantly, however, as a consequence of the continuous rotation, such antennas must necessarily define identical search and-track data rates which are multiples of the antenna scan rate. A compromise between optimum search and track data rates therefore must be made resulting in degraded system performance. The continuous antenna rotation-also limits the time of dwell on a particular azimuth and, hence, directly the systems ability to see through clutter. Moreover, the rotating antenna is normally masked to some degree in one or more azimuth angles so that full 360 coverage is not possible. In addition, separate radar subsystems require separate antennas in current applications.

SUMMARY OF THE. INVENTION In view of the foregoing, an object of the present in- .vention is to provide an improved radar system suitable for airborne early warning applications which avoids the aforementioned deficiencies of prior art systems.

In accordance with one aspect of the invention, a radar system is disclosed exhibiting'independent and optimized search and track rates. The system includes multiple array antennas comprising a fore mounted array, an aft mounted array, a port .mounted array, and a starboard mounted array, all mounted within the skin or periphery of the aircraft so as to avoid aerodynamic modifications thereto. The arrays are placed to yield full 360 coverage in azimuth and to almost completely elimminate antenna aperture blockage.

In accordance with a further aspect of the invention, a radar system is provided including multiple arrays and a digital logic means, preferably a general purpose digital computer, for defining the parameters of each beam to be tired.

In accordance with a still further aspect of the invention, the multiple arrays time share various functional circuits including an exciter, transmitter, receiver, and signal processor through switching devices.

In accordance with another significant aspect of the invention, time allocation between the multipath arrays and between operational modes such as search or track is optimized by the digital computer based on various factors such as mission objectives, current target characteristics and radar purpose.

In a preferred embodiment of the invention, a digital computer is employed to generate a radar control command to define the parameters for each beam to be fired for each of the radars subsystems since control of both radar subsystems can be performed in a nearly identical fashion, only the primary radar subsystem is described in detail herein. Typically, the control command specifies (1) array (2) beam elevation (3) beam azimuth (4) total number of fill and data pulses (5) number of fill pulses (6) frequency and (7) pulse repetition frequency. This control command is interpreted by a radar control unit which responds by controlling various system elements including the exciter, transmitter, switches, and phase shifters to cause the defined beam to be fired. Each control command also preferably specifies parameters utilized to interpret the return beam including (8) doppler offset (9) threshold (l0) range'start time (11) range end time and (I2) mode. This latter information is employed-by the radar control unit to enable primarily the receiver and signal processor to generate a beam return report which is then communicated by the radar control unit to the digital computer. The beam return report typically includes a header portion specifying the (l) clutter level and (2) jam level for each different frequencyrThe report also includes a subreport for each return beam which specifies (3) range (4) amplitude (5) doppler filter number and (6) ratio of signal to clutter plus noise. The digital computer then utilizes the beam return report to determine subsequent control commands. The computer is programmed to efficiently allocate time between various operational modes, such as search, track and verify modes in accordance with some predetermined priority criteria. Additionally, the computer can differently allocate search time between'different azimuth sectors and track time between different target tracks depending on the characteristics of the target.

BRIEF DESCRIPTION OF THE DRAWINGS In the following descriptions of the figures, figure titles which may be referred to later in the text are underlined.

FIG. la is a perspective view illustrating the preferred antenna layout in the C-1 aircraft in accordance with the present invention;

FIG. lb is a diagramatic side elevation view showing the preferred side array positioning in the C-1 aircraft;

FIG. 2a is a block diagram of an airborne early warning system incorporating a radar system in accordance with the present invention containing a primary radar subsystem and a secondary IFF/SIF radar subsystem;

FIGS. 2b and 2c are a schematic diagram of a typical array structure in accordance with the invention showing the manner in which dipole elements of primary and secondary radar antennas are interleaved;

FIG. 3a is a block diagram illustrating the primary radar subsystem of FIG. 2a in greater detail;

FIG. 3b is a schematic diagram illustrating the high power switch/duplexer of FIG. 3a in greater detail;

FIG. 4 is a block diagram of the signal processor and radar control unit utilized in the primary radar system of FIG. 3a;v

FIG. 5 is a block diagram of a digital double MTI pro cessor utilized in the signal processor of FIG. 4;

FIG. 6 is a block diagram of a doppler processor including the coherent integrator and noncoherent integrator utilized in the signal processor of FIG. 4;

FIG. 7a is a representative diagram of a storage page in the radar data buffer of the radar control unit of FIG. 4 listing the items of information contained within a radar control command and a beam return report; 

1. In an airborne radar system including a plurality of fixed beam steered antenna structures mounted in an aircraft, each directed at different sectors within a 360* azimuth volume, control means carried by said aircraft for energizing said antenna structures, said control means comprising: digital logic means for producing digital signals constituting a radar control command comprised of various information fields respectively specifying (1) an azimuth position within said azimuth volume and (2) either a track or search beam mode, said digital logic means producing successive radar control commands each specifying said search beam mode and successive azimuth positions within said azimuth volume, and said digital logic means further producing radar control commands each specifying said track beam mode and an azimuth position within said azimuth volume; said digital logic means further including priority means for determining the priorities of the radar control commands to be next produced by said search command means and said track command means; means for enabling either said search command means or said track command means dependent on the relative value of said priorities to issue an active radar control command; and transmitter means responsive to said active radar control command for supplying signal energy characteristic of the specified mode to the antenna structure corresponding to the specified azimuth sector.
 2. The system of claim 1 including: receiver means connected to said antenna structures for accepting signal energy reflected from targets; signal processing means responsive to said accepted signal energy for determining the existence and characteristics of said targets; and wherein said digital logic means includes means responsive to said determined target characteristics for computing a track update rate Delta T with respect to each target.
 3. The system of claim 2 wherein said means for computing said track update rate Delta T includes means for computing the result of the equation: Delta T 1.1 (R theta /m)1/2 where R defines the target range, theta defines the azimuth angle of the target, and m defines the tangential acceleration of the target.
 4. The system of claim 2 including memory means for storing said track update rates computed for each of said targets; and wherein said priority means includes means for determining the track priority PT for each of said targets where PT TC - TLU/ Delta T where TC represents the current real time, TLU represents the time of last update of the target data and Delta T represents the target track update rate.
 5. The system of claim 4 wherein said priority means includes means for determining the search priority PS where PS TC -TLS/ Delta TS where TC represents the current real time, TLS represents thE time of issuance of the last search radar control command and Delta TS represents the intended time between issuance of successive search radar control commands.
 6. The system of claim 5 including means for determining the value of said term Delta TS where Delta TS K.N + Delta TSN where K is a constant, N equals the number of track update rates stored in said memory means and Delta TSN is a constant representing a nominal search rate.
 7. The system of claim 6 wherein said priority means further includes: means for determining whether the largest of said track priorities PT is greater or less than 1; and being responsive to said largest track priority being less than 1 for enabling said search command means to issue a search radar control command; means responsive to said largest track priority exceeding 1 for comparing the relative manitudes of said search priority and said largest track priority for enabling said search command means if said search priority is larger and said track command means if said largest track priority is larger.
 8. The system of claim 2 wherein said priority means includes means for determining the search priority PS where PS TC -TLS/ Delta TS where TC represents the current real time, TLS represents the time of issuance of the last research radar control command and Delta TS represents the intended time between issuance of successive search radar control commands.
 9. The system of claim 2 wherein said digital logic means further includes verify command means for producing a radar control command specifying a verify beam mode and an azimuth position within said azimuth volume; and means responsive to the determination of a target existing subsequent to signal energy characteristic of said search mode being supplied to said antenna structure for enabling said verify command means.
 10. The system of claim 1 wherein said digital logic means comprises a stored program digital computer.
 11. A method of controlling an airborne radar system including a plurality of fixed beam steered antenna structures mounted in an aircraft, each directed at different sectors within a 360* surveillance volume; said method comprising the steps of: firing successive search beams, each having a particular search mode format from said antenna structure at successively different azimuth angles within said surveillance volume; monitoring the signal energy reflected to said antenna structures to determine the characteristics of reflecting targets; storing information items, each describing the characteristics of a particular reflecting target in a certain class of reflecting targets; determining the rate at which each of said stored information items should be updated; periodically firing a track beam, having a particular track mode format, from said antenna structures corresponding to each of said stored information items; and determining the priority between each of said track beams to be fired and the next successive search beam to be fired.
 12. The method of claim 11 wherein said step of monitoring to determine the characteristics of reflecting targets includes the steps of tentatively determining the location of a target and firing a verify beam, having a particular verify mode format, from said antenna structures toward said tentatively located target. 